The powering of heavy duty construction vehicles, large trucks, busses and so forth by diesel engines has presented several problems of transmission of torque from an engine, which efficiently produces power over a narrow speed range, to the drive wheels of the vehicle which of course must operate from a zero speed condition to a maximum speed condition. Consequently, transmission of the conventional gear train type are inefficient and difficult to operate when the vehicle engine is of the high power diesel type. The hydrostatic-mechanical type of transmission has been developed in order to allow vehicle engine speed to remain approximately constant through all ranges of vehicle speed. Such a transmission uses a combination of variable displacement pump-motors and a recycling planetary group driven by the vehicle engine through an input shaft, the pair of hydrostatic pumps coupled to reaction members of the recycling planetary group to provide a hydrostatic power path through the recycling group and in turn to the output gear train. Control of the displacement of the hydrostatic units and shifting of mechanical gear reduction trains in an output train provides the variable output torque. Generally speaking, one hydrostatic unit will act as a pump while the other hydrostatic unit acts as a motor to transmit power back through the recycling group. Range shifts in a mechanical gear train of the output section must occur when the elements of the output gear train are running at synchronous speed unless massive clutching members are to be included in the transmission. By synchronizing the range gear sets in the output gear train section, the designer may utilize "dog clutches" or brakes which are operated in a static condition rather than be engaged while one member is rotating at a different speed than the member to be engaged.
Hydrostatic transmissions in which the vehicle engine drives a variable displacement hydrostatic pump which in turn drives a hydraulic fixed displacement hydrostatic motor have required massive hydrostatic units in order to transmit the torque of a high power engine at low vehicle speeds directly to the vehicle wheels. Adding a conventional gear box with reduction gears permits the use of a smaller hydrostatic motor; thus the overall size of the transmission may be decreased. A limit is reached when the motor is required to operate at its maximum practical speed. These conditions define the minimum size motor of a given design that can be used in a configuration where all the power is transmitted through the hydrostatic units, with fixed-ratio gearing between the motor and the vehicle wheels. It follows that the smaller the hydrostatic unit, the lager the number of gear ranges that are required. This, in turn, increases the size of a transmission; accordingly a compromise must be reached between the size of the hydrostatic units and the reduction gear train assembly. By utilizing one variable displacement hydrostatic unit having an overcenter displacement, that is capable of reversing the fluid flow to the variable displacement unit; and, further driving the variable displacement unit directly from the input shaft gives the added capability of providing a fully hydrostatic reverse range thereby eliminating the necessity for a reverse gear train in the mechanical section.
Previous transmissions of this type have suffered disabilities in control systems, particularly in shifting at the synchronous points mentioned above. Further disabilities in control systems for transmissions of this type have been found in maintaining a constant engine speed and in limiting pressure in the hydraulic loop coupling the two hydrostatic devices. Such over-pressure in this loop occurs when an overzealous operator demands an excessive torque in acceleration or deceleration of the vehicle.